Fix running of component tests using Windows Powershell

[NyndoND]

I followed the developer guide provided in the project documentation, but when I run the command to check the code line coverage of the tests, I get the following error:
I try to run: ./gradlew componentTests
and: ./gradlew componentTests jacocoReport

Guide link I followed: https://teammates.github.io/teammates/development.html

[guilherme-puida]

This happens when running the componentTests under Powershell. The del command only exists in cmd, so it errors out when trying to delete temporary files created by the afterTestClosure in build.gradle.

A simple fix would be changing the afterTestClosure to be more shell-agnostic. I'll open a PR fixing this in a few moments :^)